1. Field of the Invention
The present invention relates to an image forming apparatus, specifically, an image forming apparatus able to easily and inexpensively suppress color deviation in color images.
2. Description of the Related Art
There are several kinds of color image forming apparatuses in the related art; one of them is the electrophotographic color image forming apparatus, which uses a semiconductor laser in its scanning optical system. In such a color image forming apparatus, a laser beam is modulated according to a color image signal generated when the original image is scanned, and is emitted to a photoconductor drum or a photoconductor belt to form latent images there; then the latent images are developed by toners having appropriate colors and are transferred to paper.
There are two types of electrophotographic color image forming apparatuses: one of them forms an image of a single color with an independent image forming unit; the other type forms color images with a single image forming unit. Generally, the former is called a four tandem engine type, because it has four image forming units corresponding to black, yellow, magenta, and cyan colors, respectively; the latter one is called a single engine type, because it uses only one image forming unit to form images combining four colors of black, yellow, magenta, and cyan.
In the single engine color image forming apparatus, a laser beam is emitted from a laser writing unit to a photoconductor belt to write latent images of black, yellow, magenta, and cyan colors in sequence on the surface of the photoconductor belt. Once a latent image having one of the above four colors is written to the photoconductor belt, this latent image is developed into a toner image immediately by a roller developer. The roller developer has four sleeves corresponding to black, yellow, magenta, and cyan colors, respectively, and a color toner corresponding to the color latent image is supplied to the photoconductor belt by a sleeve for this color.
The developed toner image is transferred from the photoconductor belt to an intermediate transfer belt, which is in constant contact with the surface of the photoconductor belt. This is the so-called “first transfer”. Usually, detectors are placed in the color image forming apparatus to detect marks acting as references on the intermediate transfer belt and output reference signals. Using the reference signals from the detectors, timing of forming latent images of different colors is unified, and latent images of different colors are formed on the photoconductor belt with the laser beam being emitted with appropriate time delays. As a result, images of different colors are formed at the same respective positions on the photo conduct belt.
A color toner image is transferred to the surface of the intermediate transfer belt by the first transfer process, and after the transfer of an image, the residual toner on the photoconductor belt is removed using a cleaner. The first transfer is repeated each time the intermediate transfer belt turns one cycle, consequently, toner images of the black, yellow, magenta, and cyan colors are transferred in sequence to the surface of the intermediate transfer belt. The toner images of the four colors are superimposed on the intermediate transfer belt, and so a color image is formed on the intermediate transfer belt.
The color image on the intermediate transfer belt is transferred to paper fed by a paper feed tray (the so-called second transfer). The image is fused onto the paper by a fuser and then the paper of the image is delivered to an output tray provided outside of the apparatus. After the transfer of images to the paper, the residual toner on the intermediate belt is removed using a cleaner.
If the color image forming apparatus is equipped with a rotational polygon mirror in the laser writing unit so as to deflect the laser beam, there may arise a problem in respect to image formation synchronization. Specifically, the intermediate transfer belt stretches because of the tolerance of the peripheral length of the intermediate transfer belt and the change of the peripheral length with time, the tolerances of the outer diameters of the rollers that firmly strains the intermediate transfer belt, and slip between the intermediate transfer belt and the rollers. Because of the stretch of the intermediate transfer belt, it is difficult to maintain the turning cycle of the intermediate transfer belt to be an integer number multiple the writing scanning interval. As a result, the scanning cycle of the laser writing unit and the turning cycle of the intermediate transfer belt are generally asynchronous, and because of this asynchronous relationship, color registration deviation occurs. Note that the problem of color registration deviation also occurs in a similar way in a color image forming apparatus using a transfer drum instead of the transfer belt.
To solve this problem, the applicants of the present invention have proposed a color image forming apparatus disclosed in Japanese Unexamined Patent Publication No. 8-152833, in which marks are formed in advance on the intermediate transfer belt, and these marks are detected by a mark detection sensor. Using the mark detection timing of the mark detection sensor as a reference, rotation of the polygon motor in the laser writing unit is controlled so as to adjust the timing of writing an image on the photoconductor belt by an appropriate period of time. In this way, color deviation in a color image is suppressed.
Further, another color image forming apparatus has also been proposed in Japanese Unexamined Patent Publication No. 10-14291, in which the driving unit of a transfer drum or a transfer belt is accelerated or de-accelerated so as to be synchronized with the writing scanning cycle; in this way, the color deviation in a color image is suppressed.
However, these color image forming apparatuses of the related art turn out to be too expensive, so a color image forming apparatus able to prevent color registration deviation in an inexpensive way and improve the quality of the image is required.
Specifically, in the color image forming apparatus disclosed in Japanese Unexamined Patent Publication No. 8-152833, the rotational speed of the polygon motor, that is, the rotational speed of the polygon mirror, is adjusted to prevent color deviation. Nevertheless, since the rotational speed of the polygon mirror is very high, it turns out to be extremely difficult from the point of view of precision to change the rotational speed while achieving synchronization, and in order to do this, a very expensive polygon motor is needed, so the total image forming apparatus may also become expensive.
In the color image forming apparatus disclosed in Japanese Unexamined Patent Publication No. 10-14291, the transfer drum or transfer belt is accelerated or decelerated so as to achieve synchronization with the writing scanning cycle, so an exclusive driving unit to do this is needed. Furthermore, since it is necessary to control speed and position of the driving unit, the driving unit, and thus the image forming apparatus, also becomes expensive.